Metallized organic polymers are utilized in numerous applications requiring conductive or reflective coatings. The primary methods for the metallization of the polymers have been vapor deposition (evaporation and sputtering) and standard or conventional electroless deposition techniques. Metallized films of polyimides (PIm) are particularly desirable in the fabrication of large-scale integrated circuits (the polyimide being primarily used as an insulating dielectric layer), flexible printed circuitry, and photovoltaic devices (primarily as a flexible substrate which can withstand the temperatures associated with the deposition of amorphous silicon).
A major concern in metallizing polyimide films, particularly for electronic applications, is the adhesion of the metal film to the polymeric substrate. It is necessary that the metal film stay well-adhered to the polymer during and after processing, which often involves electroplating and selective etching of metal film off the substrate by strong acids. This processing can lead to undercutting of metal film and loss of adhesion. Perhaps the most popular method of achieving well-adhered copper films on polyimide today is done by sputtering techniques. In this process, chromium is sputtered in the presence of oxygen onto the polyimide substrate and then copper is sputtered onto this "primed" substrate. It has been claimed that this pre-sputter with chrome in the presence of oxygen results in the covalent bonding of the chrome oxide layer to the substrate. This covalent bonding mechanism may be subject to hydrolysis reaction and may generally be expected to show reduced persistence after exposure to ambient conditions.
U.S. Pat. No. 4,459,330 discloses an electroless plating process for plating at least one main group metal on a surface of an aromatic polyimide substrate comprising the steps of forming a nonaqueous solution containing a Zintl complex, a salt or alloy of an alkali metal in a positive valence state and at least one polyatomic association of a main group metal in a negative valence state, the polyatomic main group metal being selected from the group consisting of Ge, Sn, Pb, As, Sb, Bi, Si and Te. An aromatic polymeric substrate is chosen which is reducible by the solubilized salt and is resistant to degradation during the reaction. A redox reaction is effected between the salt in solution and the substrate by contacting the solution with the substrate for a sufficient time to simultaneously oxidize and deposit the main group metal in elemental form to produce a plated substrate. The alkali metal is retained in the plated substrate, and the substrate becomes negatively charged by electrons transferred from the main group metal during the redox reaction. Only polyatomic complexes of at least seven atoms are shown.
Haushalter and Krause (Thin Solid Films, 102, 1983, 161-171 "Electroless Metallization of Organic Polymers Using the Polymer As a Redox Reagent: Reaction of Polyimide with Zintl Anions") extended the polyimide metallization discussed above to certain transition metals by using the PIm as a reducing agent toward an oxidized metal species in solution. Specifically, the treatment of PIm with methanol solutions of Zintl salts, e.g., salts of K.sub.4 SnTe.sub.4 provides a reduced intercalated material, K.sub.x PIm, with no surface metallization. The reaction of K.sub.x PIm with solutions of transition metal cations with reduction potentials more positive than that of K.sub.x PIm results in metal deposition.
The metal films deposited by this method show varied properties depending on the element and amounts deposited. For example, reaction of K.sub.x PIm with Pt.sup.2+ or Pd.sup.2+ in dimethylformamide (hereinafter DMF) rapidly gives uniform highly reflective films with conductivities approaching that of the bulk metal. In contrast, Ag.sup.+ ions, noted for their high mobility in solids, give films with resistances several orders of magnitude higher than that of palladium films containing similar amounts of metal. Apparently, the Ag.sup.+ ions can diffuse into the solid at a rate roughly comparable with the diffusion rate that the K.sup.+ and electrons exhibit in moving to the surface of the polymer (the rate of charge propogation towards the surface). The polymer is therefore partially metallized throughout the bulk solid.
Attempts to apply the above prior art to accomplish metallization of polyimide using oxidizing solutions of CuCl.sub.2.2H.sub.2 O in methanol or DMF resulted in failure. Accordingly, one aspect of the invention is a process for plating metals, and particularly transition metals, on polymeric films having electroactive centers therein, especially polyimide surfaces. A further aspect of the invention is to produce useful metallized polymeric imidecontaining substrates. Still a further aspect is to plate transition metals on polyimide having a varied thickness, and still a further object is to provide a totally additive plating process and images produced by that process.